While Global Positioning System (GPS) and Assisted Global Positioning System (A-GPS) technologies work very well for determining the location of a user outdoors, they are not reliable indoors because these technologies require an unobstructed line of sight to multiple satellites. Indoor positioning systems (IPSs) have been developed to solve this problem by utilizing networks and wireless devices to determine the locations of users inside a building. IPS technologies can be used in a variety of applications, including indoor navigation and occupancy sensing for building automation systems.
Some methods for implementing IPSs utilize radio frequency identification (RFID) or BLUETOOTH technologies. In the RFID approach, the user carries a passive or active RFID tag (e.g., embedded in his badge). Furthermore, RFID readers are installed at areas of interest in the building, such as rooms and cubicles, to read the tag information and determine which user is nearby. Similarly, in the BLUETOOTH approach, BLUETOOTH sensor nodes are deployed at areas of interest in the building and the user carries another BLUETOOTH sensor, such as a mobile device. The BLUETOOTH sensor carried by the user and the deployed sensor nodes can detect each and report the location of the user.
It is also possible to use cellular or Wi-Fi based distance approaches for determining the location of an indoor mobile device. These approaches estimate the physical distances between the mobile device and wireless access points (such as cellular base stations and Wi-Fi access points) by means such as received signal strength indication (RSSI), time of arrival (TOA), angle of arrival (AOA), and time difference of arrival (TDOA). Triangulation or trilateration techniques can then be used to estimate the location of the mobile device based on the estimated distances.